This invention relates to a method of controlling the air-fuel ratio of a mixture being supplied to internal combustion engines, and more particularly to such a control method, which is able to compensate for variations in the output characteristic of an exhaust gas component concentration sensor arranged in the exhaust system of the engine, due to tolerances in the manufacture thereof, and aging change in the output characteristic of same.
An air-fuel ratio feedback control method is generally employed which comprises comparing a value of exhaust gas component (e.g. oxygen) concentration sensed by an exhaust gas component concentration sensor (e.g. and O.sub.2 sensor) arranged in the exhaust system of the engine, with a predetermined reference value, to thereby control the air-fuel ratio of a mixture supplied to the engine to a stoichiometric mixture ratio at which the best conversion efficiency of a three-way catalyst arranged in the exhaust system is obtained, thereby enhanced the emission characteristics, etc. of the engine.
However, it is known that O.sub.2 sensors used in general for the air-fuel feedback control suffer from variations in the output characteristics due to tolerances in the manufacture thereof, and also from aging changes in the output characteristics.
If the air-fuel ratio feedback control is actually effected by using an O.sub.2 sensor having an output characteristic that the output voltage is apt to be biased toward a rich side, the resulting air-fuel ratio is controlled to a leaner value than the stoichiometric mixture ration, whereas if the control is effected by using an O.sub.2 sensor having an output characteristic that the output voltage is apt be biased toward a lean side, the resulting air-fuel ratio is controlled to a richer value than the stoichiometric mixture ratio.
In order to eliminate the deviation of the air-fuel ratio caused by variations of aging changes in the output characteristics of O.sub.2 sensors, methods of selecting O.sub.2 sensors have been proposed e.g. by Japanese Provisional Patent Publications (Kokai) Nos. 62-93644 and 62-119450, in which respective output characteristics of O.sub.2 sensors are checked by means of a special checking device before installing the sensors in vehicles, and based upon the checking results, the sensors are classified into those whose output voltage are apt to be biased toward a rich side and those toward a lean side.
However, the classification or assortment requires such time and labor, resulting in degraded productivity. Meanwhile, if no measures is taken to prevent aging change in the output characteristic of the O.sub.2 sensor, the resulting air-fuel ratio will be biased to the lean or rich side, thereby leading to degraded drivability, increased fuel consumption, and deteriorated emission characteristics of the engine.